The invention relates to a system and method for maintaining a set point temperature in a transport temperature control system; and more particularly the invention relates to a temperature control system and method for efficiently obtaining and maintaining a predetermined set point temperature in a conditioned space by monitoring the difference between the set point and conditioned space temperatures, and also monitoring the time rate of change of the temperature of the conditioned space return air.
Mobile temperature control systems maintain air in a conditioned space at a predetermined set point temperature required to keep the goods in the conditioned space suitable for delivery to an end user or customer. The conditioned space may be a container, truck or tractor trailer that is transported to one or more delivery locations.
During shipment of the goods, the temperature of the air in the conditioned space typically fluctuates due to changing ambient conditions, opening and closing the doors to the conditioned space during deliveries, and the type of goods in the conditioned space. As a result, in order to maintain the conditioned space temperature at the predetermined acceptable set point temperature, it is usually necessary to frequently switch the unit between cooling and heating modes. For example, if the air temperature in the conditioned space falls below the desired set point temperature, the temperature control system will switch from a cooling mode, where the system delivers air to the conditioned space at a temperature that may be ten degrees below the temperature of the conditioned space air, to a heating mode, where the system delivers air to the conditioned space at a temperature that may be ten degrees warmer than the temperature of the air in the conditioned space. Conversely, if the actual conditioned space air temperature is above the predetermined set point temperature, the temperature control system will switch from the heating mode to the cooling mode.
Switching between the temperature control unit heating and cooling modes to obtain the set point temperature in the conditioned space often causes the set point temperature to be overshot. After the heating mode is terminated, the resultant conditioned space air temperature usually overshoots the set point temperature and settles at a temperature above set point; and conversely, after the cooling mode is terminated, the resultant conditioned space air temperature usually overshoots the set point temperature and settles at a temperature that is below the desired set point temperature. Such deviations from the set point temperature can negatively affect the quality of the goods in the conditioned space.
Conventional temperature control units are driven by a prime mover that operates between a low speed of 1450 rpm and a high speed of 2200 rpm. Typical temperature control units operate in low speed cooling mode, low speed heating mode, high speed cooling mode and high speed heating mode. The required operating mode is dependant on the magnitude of the difference between the set point and actual temperature of the conditioned space air, and also whether the actual conditioned space temperature is below or above the set point temperature. The units are operated in the mode required to return the conditioned space to the set point temperature and the unit typically operates in the prescribed mode until the temperature returns to the set point temperature. In most instances temperature control units effectively maintain the conditioned space set point temperature however in some instances it is possible for the temperature in the conditioned space to hang at a temperature outside of the set point temperature for a period of time. In systems that experience hanging, the mode of operation of the unit is typically determined simply by calculating the difference between the set point temperature and the temperature of the air in the conditioned space and selecting the mode of operation that corresponds to the temperature difference. This is an inefficient method for maintaining set point temperature in the conditioned space and decreases the fuel efficiency of the unit.
In order to overcome the problem of hanging in conventional temperature control units, temperature control units now may include timers that are programmed to run the unit at either high or low speed cooling or heating modes for a period of time and when the timer times out, if the conditioned space air has not returned to the desired set point temperature, the controller then switches to the next higher mode, such as high speed cooling or heating mode. However, automatically switching the unit from one operating speed to another when a timer times out is also an inefficient method of operating the temperature control system. The timers do not take into account the rate of change of the conditioned space air and also do not take into account the proximity of the temperature of the conditioned space air relative to the set point temperature when the timer times out. The unit may be switched to high speed when the set point could have been reached if the unit remained at the low speed for an additional period of time. Additional fuel is expended switching the unit operating modes.
The foregoing illustrates limitations known to exist in present devices and methods. Thus, it is apparent that it would be advantageous to provide an alternative directed to efficiently obtaining and maintaining the set point temperature. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.